The cryogenic rectification of air to produce oxygen and nitrogen is a well established industrial process. Typically the feed air is separated in a double column system wherein nitrogen shelf or top vapor from a higher pressure column is used to reboil oxygen bottom liquid in a lower pressure column.
The demand for lower purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Less vapor boilup in the stripping sections of the lower pressure column, and less liquid reflux in the enriching sections of the lower pressure column are necessary for the production of lower purity oxygen which has an oxygen purity of 97 mole percent or less, than are typically generated by the operation of a double column.
Accordingly, lower purity oxygen is generally produced in large quantities by a cryogenic rectification system wherein feed air at the pressure of the higher pressure column is used to reboil the liquid bottoms of the lower pressure column and is then passed into the higher pressure column. The use of air instead of nitrogen to vaporize the lower pressure column bottoms reduces the air feed pressure requirements, and enables the generation of only the necessary boil-up in the stripping sections of the lower pressure column either by feeding the appropriate portion of the air to the lower pressure column reboiler or by partially condensing a larger portion of the total feed air.
While the conventional air boiling cryogenic rectification system has been used effectively for the production of lower purity oxygen, its ability to generate reflux for supply to the top of the lower pressure column is limited. This results from the fact that condensation of some of the feed air reduces the available vapor for generation of nitrogen reflux in the higher pressure column. More power is consumed because oxygen recovery is reduced as a result of the reduced capability to generate reflux.
Accordingly, it is an object of this invention to provide a cryogenic rectification system for producing lower purity oxygen which employs a double column arrangement and which operates with reduced power requirements over that of conventional systems.